So in the early 19th century they used to think Swansea got its name from an abundance of porpoises in the bay. I've never seen a porpoise in the bay but it would be neat.

(The current thinking is that it's from Old Norse, named after someone called Svein or Sweyn.)

https://archive.org/details/topographicaldic12lewi/page/n365/mode/2up